Remove unused code for multiple-point ECC multiplication.
The points are only converted to affine form when there are at least
three points being multiplied (in addition to the generator), but there
never is more than one point, so this is all dead code.
Also, I doubt that the comments "...point at infinity (which normally
shouldn't happen)" in the deleted code are accurate. And, the
projective->affine conversions that were removed from p224-64.c and
p256-64.c didn't seem to properly account for the possibility that any of
those points were at infinity.
Change-Id: I611d42d36dcb7515eabf3abf1857e52ff3b45c92
Reviewed-on: https://boringssl-review.googlesource.com/7100
Reviewed-by: David Benjamin <davidben@google.com>